1) Field of the Invention
The present invention relates to a method and an apparatus for controlling Raman amplification of optical signals.
2) Description of the Related Art
Recently, distributed-parameter-type optical fiber amplification called Raman amplification is receiving attention in the field of optical communications. In the Raman amplification, a physical phenomenon in which vibrations of materials inelastically scatter incident light so as to produce Raman scattered light having a wavelength which is different from the wavelength of the incident light is utilized, and strong excitation light is injected into an optical-fiber transmission line so that optical amplification occurs in the entire optical-fiber transmission line.
The gain of the Raman amplification has a peak at a wavelength which is about 100 nm longer than the wavelength of incident excitation light. That is, the Raman amplification of optical signals having a wavelength about 100 nm longer than the wavelength of the incident excitation light is most efficient. For example, in order to amplify an optical signal having a wavelength of 1.55 micrometers, excitation light having a wavelength of 1.45 micrometers is injected into the optical-fiber transmission line.
When repeaters are arranged to realize the Raman amplification, a longer optical fiber cable can be laid, and intervals between repeaters can be increased. Thus, the Raman amplification is very beneficial to high-speed, large-capacity optical transmission.
In addition, with the recent increase in the transmission rate corresponding to the rapid development of the Internet, use of WDM (Wavelength Division Multiplex) transmission is rapidly spreading. In the WDM transmission, a plurality of optical signals having a plurality of different wavelengths is concurrently transmitted through a single optical fiber by multiplexing the plurality of optical signals.
When the Raman amplification is used in WDM transmission systems, the width of a gain band which can be realized by exciting an optical fiber with incident excitation light having a single wavelength becomes smaller than the widths of the bandwidths used in the current WDM systems. Therefore, normally, the width of the gain band is expanded by providing a plurality of excitation light sources and exciting the optical fiber with a plurality of excitation wavelengths (e.g., six to eight excitation wavelengths) of excitation light. For example, in order to amplify broadband signal light in broadband WDM systems, Raman amplification is performed by using excitation light having a plurality of wavelengths.
However, in the conventional WDM systems, the Raman amplification is performed by concurrently injecting a plurality of wavelengths of continuous excitation light emitted from a plurality of excitation light sources. Therefore, the excitation light is strongly affected by nonlinear optical phenomenons which are specific to excitation with a plurality of wavelengths of excitation light, such as the stimulated Raman scattering (SRS) and the four wave mixing (FWM), as explained below.
In general, the SRS is a phenomenon in which energy is exchanged through optical, lattice vibration in an optical fiber between a plurality of wavelengths of light propagated through the optical fiber. In the SRS, light having a longer wavelength is Raman amplified with light having a shorter wavelength. For example, the Raman amplification utilizes the SRS for amplification of signal light.
When the SRS occurs between different wavelengths of light, a portion of energy of light having a shorter wavelength (i.e., higher photon energy) is transferred to light having a longer wavelength (i.e., lower photon energy). Therefore, when a plurality of different wavelengths of light pass through an optical fiber, the powers of the plurality of different wavelengths of light become different at the output end of the optical fiber even when the powers of the plurality of different wavelengths of light are initially identical at the input end of the optical fiber. For example, the power spectrum of the light at the output end of the optical fiber exhibits a tilt as illustrated in FIG. 1.
The amount T(dB) of the tilt can be expressed as                               T          =                                                10              ⁢                                                           ⁢              log              ⁢                                                           ⁢              10              ⁢                                                1                  +                                                            N                      ⁡                                              (                                                  N                          -                          1                                                )                                                              ⁢                    P                    ⁢                                                                                   ⁢                    Δ                    ⁢                                                                                   ⁢                    f                    ⁢                                                                                   ⁢                                          γ                      p                                        ⁢                                                                                            L                          eff                                                /                                                  A                          eff                                                                    /                      6                                        ×                                          10                      13                                                                                        1                  -                                                            N                      ⁡                                              (                                                  N                          -                          1                                                )                                                              ⁢                    P                    ⁢                                                                                   ⁢                    Δ                    ⁢                                                                                   ⁢                    f                    ⁢                                                                                   ⁢                                          γ                      p                                        ⁢                                                                                            L                          eff                                                /                                                  A                          eff                                                                    /                      6                                        ×                                          10                      13                                                                                                                      ,                            (        1        )            where N is the number of channels, P is the amount of input power, Δ f is the channel spacing, Leff is the effective length of the optical fiber, γp is the peak gain coefficient, and Aeff is the effective cross-sectional area of the fiber core.
FIG. 1 is a graph indicating simulated values of line reception levels of light having various wavelengths at input and output ends of an optical fiber. In FIG. 1, the abscissa indicates the wavelength(nm), and the ordinate indicates the line reception level(dBm). In the example of FIG. 1, simulated values of line reception levels of light having six wavelengths of 1,430, 1,435, 1,455, 1,460, 1,480, and 1,485 nm at input and output ends of an optical fiber are indicated. That is, the minimum wavelength is 1,430 nm, and the maximum wavelength is 1,485 nm.
As illustrated in FIG. 1, the simulated values of line reception levels of light at the output end of the optical fiber exhibit a tilt of about 0.12 dBm, while the simulated values of line reception levels of light at the input end of the optical fiber do not exhibit a tilt.
As mentioned before, in the conventional WDM systems, the Raman amplification is performed by concurrently injecting into an optical fiber transmission line a plurality of wavelengths of continuous excitation light emitted from a plurality of excitation light sources. Therefore, the SRS also occurs between different wavelengths of excitation light. In this case, a portion of power of excitation light having a shorter wavelength is used for Raman amplifying excitation light having a longer wavelength, and thus power of a channel which is Raman amplified with the excitation light having the shorter wavelength. Thus, the powers of signal light in the respective channels become different, and the efficiency in utilization of the excitation light having a shorter wavelength is lowered.
On the other hand, the FWM is a phenomenon in which new frequencies w3 and w4 of light are generated through cubic nonlinear polarization caused by injection of two frequencies w1 and w2 of light. That is, in the FWM, new interference light is generated in an optical fiber by interference between a plurality of different wavelengths of light propagating through the optical fiber.
When a material is irradiated with light, electrons in atoms or molecules are displaced by electric fields of the light, i.e., polarization occurs. When a material is irradiated with strong laser light, the material exhibits a nonlinear behavior, i.e., second-order or third-order nonlinear polarization occurs, where the second-order or third-order nonlinear polarization is proportional to squares or cubes of electric fields of the light.
The FWM is most enhanced when an input wavelength coincides with the zero-dispersion wavelength of the optical fiber. Usually, the wavelengths of excitation light used in Raman amplifiers are in the 1,400 nm range. Therefore, when an optical fiber has a zero-dispersion wavelength in the 1,400 nm range, crosstalk caused by the four wave mixing between different wavelengths of light increases. When crosstalk increases, transmission quality deteriorates. In addition, since a portion of the excitation power is used for producing the interference light, the excitation efficiency is reduced. The amount PFWM of crosstalk can be expressed by the formula (2d), where Δβ is expressed by the formula (2a), K is expressed by the formula (2b), and ηijk is expressed by the formula (2c).                                                                         Δ                ⁢                                                                   ⁢                β                            =                            ⁢                                                β                  ijk                                +                                  β                  k                                -                                  β                  i                                -                                  β                  j                                                                                                        =                            ⁢                                                (                                                            π                      ⁢                                                                                           ⁢                                              λ                        4                                                                                    3                      ⁢                                              c                        2                                                                              )                                ⁢                                  (                                                            ⅆ                      Dc                                                              ⅆ                      λ                                                        )                                ⁢                                  {                                                                                    (                                                                              f                            ijk                                                    -                                                      f                            0                                                                          )                                            2                                        -                                                                  (                                                                              f                            i                                                    -                                                      f                            0                                                                          )                                            2                                        -                                                                  (                                                                              f                            j                                                    -                                                      f                            0                                                                          )                                            2                                        +                                                                                                                                        ⁢                                                (                                                            f                      k                                        -                                          f                      0                                                        )                                2                            }                                                          (2a)                                K        =                  32          ⁢                                                    π                2                            ⁡                              (                                                      L                    eff                                    /                                      A                    eff                                                  )                                      /                          (                                                n                  2                                ⁢                λ                            )                                                          (2b)                                          η          ijk                =                              (                                          α                2                                                              α                  2                                +                                  Δ                  ⁢                                                                           ⁢                                      β                    2                                                                        )                    ⁢                      (                          1              +                                                4                  ⁢                                      exp                    ⁡                                          (                                                                        -                          α                                                ⁢                                                                                                   ⁢                        L                                            )                                                        ⁢                                                            sin                      2                                        ⁡                                          (                                              Δ                        ⁢                                                                                                   ⁢                        β                        ⁢                                                                                                   ⁢                                                  L                          /                          2                                                                    )                                                                                                            {                                          1                      -                                              exp                        ⁡                                                  (                                                                                    -                              α                                                        ⁢                                                                                                                   ⁢                            L                                                    )                                                                                      }                                    2                                                                                        (2c)                                          P          FWM                =                              P            ijk                    =                                    η              ijk                        ⁢                                          K                2                            ⁡                              (                                  D                  χ                                )                                      ⁢                          (                                                                    P                    i                                    ⁡                                      (                    0                    )                                                  ⁢                                                      P                    j                                    ⁡                                      (                    0                    )                                                  ⁢                                                      P                    k                                    ⁡                                      (                    0                    )                                                  ⁢                                  exp                  ⁡                                      (                                                                  -                        α                                            ⁢                                                                                           ⁢                      L                                        )                                                                                                          (2d)            
In the formulas (2a), (2b), (2c), and (2d), n0 is the refractive index of a fiber core, λ is the wavelength, c is the velocity of light, D is the degeneracy factor, α is the fiber attenuation coefficient, β is the propagation constant, χ is the third-order nonlinear susceptibility, Aeff is the effective cross-sectional area of the fiber core, L is the fiber length, Leff is the effective length of the optical fiber (i.e., Leff=(1−exp(−αL))/α), f0 is the frequency corresponding to the zero-dispersion wavelength, Dc is the fiber chromatic dispersion, fi, fj, and fk are frequencies of excitation light and signal light, Pi (0), Pj (0), and Pk (0) are input powers of the frequencies fi, fj, and fk of excitation light and signal light, and fijk is a frequency of newly generated light.
FIGS. 2(A) and 2(B) are graphs indicating simulated values of crosstalk at various frequencies in the first case where the FWM does not occur and in the second case where the FWM occurs. In each of FIGS. 2(A) and 2(B), the abscissa indicates the wavelength(nm), and the ordinate indicates the amount of crosstalk(dB). In the example of FIG. 2(A), simulated values of the crosstalk of light at eight frequencies ranging from the minimum frequency of 1,420 nm at the intervals of 10 nm are indicated, and the zero-dispersion wavelength of the optical fiber is 1,451.9 nm.
The amounts of crosstalk in the first case where the FWM does not occur are in the range between −40 and −38 dB as illustrated in FIG. 2(A), and the amounts of crosstalk in the second case where the FWM occurs are in the range between −13.5 and −11 dB as illustrated in FIG. 2(B). That is, the amounts of crosstalk are increased by the FWM.
As explained above, since a plurality of wavelengths of continuous excitation light emitted from a plurality of excitation light sources is concurrently injected in an optical fiber in the conventional Raman amplified WDM systems, optical transmission is affected by nonlinear optical phenomenons including the SRS and the FWM.
That is, the SRS occurs between a plurality of different wavelengths of excitation light, and therefore differences arise between the powers of the plurality of different wavelengths of excitation light. Thus, differences also arise between the powers of a plurality of wavelengths of signal light which are respectively excited with the plurality of different wavelengths of excitation light.
According to a conventional technique, initial power differences are provided to the plurality of different wavelengths of excitation light so as to cancel the differences caused by the SRS, i.e., initial power differences realizing an inverse profile to the profile of the line reception levels at the output end of the optical fiber as illustrated in FIG. 1 are provided to the plurality of different wavelengths of excitation light when the plurality of different wavelengths of excitation light is injected into the optical fiber. However, it is impossible to quantitatively determine and set the above initial power differences for each system. Even when the above initial power differences can be provided, the excitation light is not efficiency utilized.
In addition, since the FWM occurs in the conventional Raman amplified WDM systems, the amount of crosstalk increases, and a portion of the excitation power is used for producing the interference light. Therefore, the transmission quality deteriorates.
As explained above, in the conventional Raman amplified WDM system, a plurality of wavelengths of continuous excitation light is concurrently injected into an optical transmission line, and therefore the SRS and the FWM are caused by the plurality of wavelengths of excitation light. Thus, the efficiency in Reman amplification is reduced, and transmission quality deteriorates.